Conception Date Calculator
Enter your due date (or last period) to estimate when conception happened.
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Your results
Conception Date Calculator by TheCalcTools
How the Conception Date Calculator works
A pregnancy is dated as 280 days (40 weeks) from the first day of the last period, and conception usually happens around ovulation, about two weeks after that period starts in a 28-day cycle. So conception is roughly 266 days before the due date.
Sperm can live up to 5 days, and ovulation timing varies, so the real date can differ by several days. An early ultrasound dates a pregnancy more accurately than dates alone.
The formula
Worked example
Using the default values shown in the calculator:
| I know my | Due date |
|---|---|
| Due date | 2027-06-20 |
| First day of last period | 2026-09-13 |
| Cycle length (days) | 28 |
- Due 20 June 2027: conception most likely around Sunday, 27 September 2026 (24 September 2026 to 30 September 2026).
Result:
| Most likely conception date | Sunday, 27 September 2026 |
|---|---|
| Possible range | 24 September 2026 to 30 September 2026 |
| Due date | 20 June 2027 |
| Implied first day of last period | 13 September 2026 |
Frequently asked questions
Can this tell me who the father is?
No. The uncertainty of several days makes date-based estimates unreliable for that. Only a DNA test can establish paternity.
Sources and further reading
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This calculator gives general estimates for information and education. It is not medical advice and does not diagnose anything. Talk to a qualified health professional about your own situation. See our disclaimer.